The present invention relates to an optical waveguide element for use in an optical integrated circuit, a method of manufacturing such an optical waveguide element, and an optical coupler which employs such an optical waveguide element for coupling optical power.
Optical integrated circuits include an optical waveguide element comprising an optical waveguide formed by a portion with a slightly higher refractive index on the surface of a single substrate. An optical integrated circuit also contains various other optical components such as a laser diode as a light source, a switch, a modulator, a photodiode as a photodetector, and others. The integration of such an optical circuit makes an optical system incorporating the same, small in size, lightweight, stable in operation, and high in performance.
An optical waveguide element comprises a dielectric substrate capable of transmitting light therethrough, typically made of glass, and a waveguide portion in the form of film formed by deposition or diffusion on the surface of the substrate, the waveguide portion having a slightly high refractive index and a thickness as small as the wavelength of light to be passed through the waveguide portion. Light applied to the optical waveguide element is confined in and guided through the high-refractive-index waveguide portion.
One known optical waveguide element includes, as shown in FIG. 45 of the accompanying drawings, an optical waveguide 410 formed by an ion exchange process (proton exchange process) replacing Li.sup.+ with H.sup.+ in the crystal of an optical material 400 such as lithium niobate (LiNbO.sub.3). Another optical waveguide element has an optical waveguide 410 formed in the optical material 400 by diffusing titanium oxide (TiO.sub.2) which serves to increase the refractive index of the optical material 400. A method of fabricating an optical waveguide element based on the proton exchange process and such an optical waveguide element are disclosed in NIKKEI ELECTRONICS, page 90, line 3 to 10, published July 14, 1986.
According to another known fabrication method, an optical material as shown in FIG. 46 is produced, thereafter magnesium oxide (MgO) which reduces the refractive index of an optical material 420 is diffused into the optical material 420 to produce therein an optical waveguide 430 having a higher refractive index than that of the surrounding optical material 420.
FIG. 47 shows an end coupling method for applying a light wave to or emitting a light wave from an optical waveguide element. An end face of a substrate 510 having an optical waveguide 500 is ground, and a light wave is converged by a light converging means 520 such as a condenser lens and applied directly to the optical waveguide 500 through the ground end face.
FIG. 48 illustrates a prism coupling method which couples optical power through the use of a prism 530 such as a rutile prism having a higher refractive index than that of an optical waveguide 500 formed in a substrate 510. The excitation of a guided wave is effected by distribution matching between the applied or emitted wave and a guided mode.
Another known method is a tapered coupling method as shown in FIG. 49 which employs an optical waveguide 500 including a tapered end portion 540 progressively varying in thickness. A light wave guided by the waveguide 500 partly passes through the boundary between the waveguide 500 and a substrate 510 and radiates into the substrate 500. The amount of a light wave entering the substrate 500 progressively increases toward the tip end of the tapered portion 540. The guided wave which has traveled through the waveguide 500 is cut off and caused to radiate into the substrate 510 at a position where the thickness of the waveguide 500 is of a certain reduced value. It is possible to couple an output light beam with an optical fiber 550, as indicated by the broken lines, by utilizing the waist of the light beam.
FIGS. 50 and 51 illustrate a grating coupling method in which a grating coupler 570a comprising diffraction grating slits on the surface of a substrate 560 is employed to effect conversion between a guided mode in which a light wave is guided through a waveguide layer 570 and a radiation mode in which the guided wave is emitted out of the waveguide layer 570, for coupling optical power.
A tapered waveguide may be fabricated by a method as shown in FIG. 52. A mask 580 is positioned parallel to and spaced a small distance from a substrate 510 in partly over-lapping relation thereto. Particles 600 of the material forming an optical waveguide film are then applied perpendicularly to the substrate 510 by sputtering or evaporation. A tapered end portion 540 of the waveguide is formed by material particles 600 which enters for step converge into a masked area 590 below the mask 580.
The end coupling method as shown in FIG. 47 is disadvantageous in that a light beam to be applied to the waveguide has to be positionally adjusted in the order of several microns outside of the waveguide into alignment with the waveguide which is 2 to 5 .mu.m thick, and the end face of the waveguide has to be ground for required flatness.
The prism coupling method as shown in FIG. 48 allows relatively easy coupling of optical power, but has problems in that it requires fine adjustment of the gap and the beam applying position, is poor in stability, and needs an expensive prism of high refractive index and high precision and adjustment mechanism of prism.
According to the tapered coupling method as shown in FIG. 49, since the waveguide has a flat tapered shape with the thickness linearly varying in the direction in which the wave is transmitted, the output beam from the waveguide is large in diameter, and also since the beam diameter progressively increases as it goes away from the waveguide, the coupling efficiency between the optical waveguide and another optical element such as an optical fiber is low.
According to the grating coupling method as shown in FIG. 50, if the grating coupler 570a (FIG. 49) has to have a light converging ability, the grating coupler has to be a chirped grating in which the distance between adjacent grating slits is progressively smaller. FIG. 51 shows such a chirped grating coupler. The grating coupler comprises a substrate 560 of silicon (Si), a buffer layer 610 of silicon oxide (SiO.sub.2) deposited on the substrate 560 up to a thickness of 1.86 .mu.m, a waveguide 570 of glass #7059 manufactured by Corning, U.S.A., which is deposited on the buffer layer 610 up to a thickness of 0.95 .mu.m, and a cladding layer 620 of silicon nitride (SiN) deposited on the waveguide 570 up to a thickness of 0.035 .mu.m. With the distance between adjacent grating slits being progressively vaired from 0.75 .mu.m to 0.52 .mu.m for a length of 1.0 mm, a light beam having a wavelength of 590 nm is converged at a point in space which is 2.0 mm away from the grating coupler. The grating coupler has to be manufactured using an electron beam printing process which produces such closely spaced grating slits. The efficiency of light utilization of the grating coupler is low, about 50%.
A laser beam, for example, is applied to an optical waveguide element through either a glass fiber bonded to the end face of the optical waveguide element or an optical lens located near the end face of the optical element.
Accordingly, the end face of the optical waveguide element must be of a mirror finish. If an edge of the end face of the optical waveguide element were broken away or round, it would cause dispersion or refraction of light, resulting in a loss of light between the glass fiber and the optical waveguide element. It would highly be difficult to fabricate an optical waveguide element without breaking away or rounding an edge of the end face thereof.